1. Field of the Invention
This invention relates to a method for the manufacture of a precious metal product using a clayish composition based on a precious metal powder and more particularly to a method for the manufacture of a precious metal product, which method is capable of fixing an accessory such as a gem or a metallic part such as a creaser or loket bails accurately at an intended position with ample strength to a matrix.
2. Prior Art Statement
Clayish compositions containing a precious metal powder are easy to form and, therefore, are manufactured in various shapes and designs such as pendant tops, rings, brooches, and pierces, and fired and marketed as precious metal goods. As a means to integrate such an accessory as a gem with a precious metal product, the method disclosed in Japanese Patent Public Disclosure Hei 04-146606 is known. This method accomplishes the integration by forcing an accessory made of a material having a higher melting point than the precious metal powder into a formed mass of a clayey composition in a plastic state until it is set in place therein and firing the resultant composite thereby enabling the formed mass to shrink and take firm hold of the accessory with powerful constraining force.
This method, therefore, obviates the necessity of relying on such a highly expert technique as "brazing" or "caulking" as has been heretofore adopted in the fixation of an accessory and also enjoys the advantage of ensuring manufacture of a product free of discoloration, a fault apt to arise in brazing.
Since the clayish composition used in this method contains a large amount binder, however, it is destined to exhibit a large final shrinkage falling in the range of 15-26% upon sintering. Depending on the volume and shape of the formed mass, therefore, this method is incapable of enabling the accessory fixed in the product finished by the sintering to be retained accurately at the target position.
Even when a gem as an accessory is forced into an undried formed mass until it is accurately set in place therein, once sintered, the table (the smooth central part) of the gem in the resultant composite is liable to tilt, possibly to the extent of causing the gem to rise above or sink under the desired position. Moreover, the degree with which the final seat of the gem deviates from the target position tends to grow in direct proportion to the variation in wall thickness of the formed mass, the ruggedness of the surface of the formed mass, and the number of accessories to be set.
This deviation in position is thought to occur because the formed mass which theoretically ought to shrink evenly actually develops strain or internal stress, fails to shrink evenly, and consequently deforms because the wall thickness thereof locally varies or because the shrinkage thereof due to drying or sintering proceeds earlier near the surface thereof than in the interior.
Further, since the method mentioned above causes the gem to set firmly in position in the sintered composite owing to the voluminal contraction occurring during sintering, the inaccurately mounted gem can no longer be adjusted in orientation or be removed.
The method also requires the accessory to withstand a temperature higher than the sintering temperature of the precious metal powder. It, therefore, cannot be safely applied to such accessories as pearl and amber which are thermally decomposed below the sintering temperature or to natural precious stones or semi-precious stones which may be discolored or fractured when heated to elevated temperatures.